I have never heard of that problem....has your mechanic worked on many DMaxs? I would be willing to bet it is your injectors gone south. Early LB7s are known for this. Do you have diesel fuel in the oil? If so, take it to a GM dealership and have the injectors replaced. It is free and injectors are covered under special policy 04039 for 7yr/200,000 mile warranty no deductable. Of course if you are over 200,000 miles, then it probably won't be covered.